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Astable Multivibrator with the 555 Timerthe state of the SR flip−flop whose output is either Q or Q. Thus, if Q is High (Set state), Q isLow, and if Q is Low, Q will be High (Reset state) or vice versa.The SR flip−flop is Set when a High level is applied to the S input, and it is Reset when a Highlevel is applied to the R input. Accordingly, the flip−flop is Set or Reset depending on the outputsof the two comparators, and these outputs are determined by the inputs Threshold at the plus (+)input of Comparator 1, and Trigger at the minus (−) input of Comparator 2. The output of the 555Timer is the Q output of the SR flip−flop and when it is Low, Q will be High, and if the input Dischargeis High, the transistor will be saturated and it will provide a path to the ground.7.3 Astable Multivibrator with the 555 TimerA useful application of the 555 Timer is as an astable multivibrator. From our previous discussion,we recall that the astable multivibrator is a pulse generator producing waveforms such as thatshown in Figure 7.5.T 1T 2TFigure 7.5. Typical waveform for an astable multivibratorPeriod T T 1 T 2For the waveform of Figure 7.5,= =+Pulse Repetition Frequency= f =1--TDuty CycleFigure 7.6 shows an astable multivibrator employing a 555 Timer. We will see that with this circuitthe duty cycle will always be greater than 0.5 as shown in Figure 7.7.For the circuit of Figure 7.6 let us first assume that the capacitor is uncharged and the SR flip−flopis Set. In this case the output is High and the transistor is does not conduct. The capacitor thenwill charge towards V CC through the resistors R A and R B . When the capacitor reaches the valuev C = 1⁄ 3 V CC the output of Comparator 2 goes Low and the SR flip−flop remains Set.=T----- 1TElectronic Devices and Amplifier Circuits with MATLAB Computing, Second EditionCopyright © Orchard Publications7−3